The present invention relates to a method and apparatus for removing impurities from a feed gas stream by adsorption, particularly by temperature swing adsorption.
The cryogenic purification of air requires a pre-purification step for the removal of high-boiling and hazardous materials. Principal high-boiling air components include water and carbon dioxide. If removal of these impurities from ambient air is not achieved prior to the air separation system, then water and carbon dioxide will freeze out in cold sections of the separation apparatus (for example in the heat exchangers and liquid oxygen sump) causing pressure drop and operational problems. Various hazardous materials including nitrous oxide, acetylene, and other hydrocarbons also must be removed. High-boiling hydrocarbons are problematic because they concentrate in the liquid oxygen section of the separation apparatus, resulting in a potential explosive hazard. In addition, nitrous oxide can form unstable compounds with the hydrocarbons and this is another potential hazard.
Adsorption processes are generally preferred for the removal of these impurities from feed air to cryogenic air separation plants. These adsorption processes include thermal swing adsorption (described in U.S. Pat. Nos. 4,541,851 and 5,137,548) and pressure swing adsorption (described in U.S. Pat. No. 5,232,474) systems. These systems usually are designed for total water and carbon dioxide removal from ambient air. Adsorbents selective for water and carbon dioxide are required for these systems.
Thermal swing adsorption processes typically use layered adsorbent beds in which the feed air first contacts a water-selective adsorbent such as alumina or silica gel. The dry, carbon dioxide containing air then contacts a zeolite adsorbent to remove carbon dioxide to very low levels. Hydrocarbons and nitrous oxide also are removed by the appropriate adsorbents, typically in layered configuration.
The term xe2x80x9cmass transfer zonexe2x80x9d as used herein refers to the section of an adsorbent bed in which adsorbent loading of the adsorbate is occurring. Ahead of the leading edge of the mass transfer zone, the gas concentrations of the adsorbed components are reduced relative to the feed. At the trailing edge of the mass transfer zone and behind the trailing edge gas phase composition is substantially equal to that of the feed mixture and the adsorbent is substantially loaded to capacity with the adsorbed components from the feed mixture. A small mass transfer zone is beneficial and allows a higher adsorbate loading on the adsorbent before the leading edge of the mass transfer zone breaks through the effluent end of the adsorbent bed. This results in more efficient adsorbent bed operation. Consequently, a smaller bed may be used or the onstream time between regenerations may be increased.
Small adsorbent particles generally provide shorter mass transfer zones than large adsorbent particles. The prior art processes disclosed in U.S. Pat. Nos. 4,964,888 and 5,728,198 and European Patent Publication EP-A-449576 improve mass transfer characteristics by using a layer of smaller adsorbent particles downstream of a layer of larger adsorbent particles. However, the use of small adsorbent particles can cause a greater pressure drop across the adsorbent bed and can result in fluidization, adsorbent attrition, and carryover of fine adsorbent particles. The present invention, which is described below and defined by the claims which follow, addresses this problem by using at least two layers of different adsorbents which have substantially the same average particle diameters. The relative adsorption selectivities of the adsorbents for two of the impurities are specified as described below.
The invention relates to a method for removing a first and a second minor component from a gas mixture comprising the first and second minor components and one or more major components. The method comprises the steps of:
(a) providing a first adsorbent zone containing a first adsorbent material and a second adsorbent zone containing a second adsorbent material, wherein the selectivity of the first adsorbent material for the first minor component relative to the second minor component is greater than the selectivity of the second adsorbent material for the first minor component relative to the second minor component, and wherein the average particle diameter of the first adsorbent material and the average particle diameter of the second adsorbent material are substantially the same;
(b) passing the gas mixture comprising the first and second minor components and the one or more major components through the first adsorbent zone and subsequently through the second adsorbent zone; and
(c) withdrawing from the second adsorbent zone a purified gas containing the one or more major components and depleted in the first and second minor components.
The first minor component may be nitrous oxide and the second minor component may be carbon dioxide. The one or more major components may comprise oxygen and nitrogen. The gas-mixture may comprise air.
The average particle diameter of the first adsorbent material preferably is between about 85% and about 115% of the average particle diameter of the second adsorbent material. The average particle diameter of the first adsorbent material may be between about 0.5 mm and about 5 mm. The first adsorbent material may comprise CaX zeolite and the second adsorbent material may comprise 13X zeolite.
The mixture may further comprise water and an additional adsorbent zone may be provided prior to the first adsorbent zone, and this additional adsorbent zone preferably contains adsorbent material which selectively adsorbs water from the gas mixture prior to the first adsorbent zone.
The gas mixture typically is provided at a temperature between about 0xc2x0 C. and about 50xc2x0 C. The method may further comprise terminating steps (b) and (c) and regenerating the first and second adsorbent materials by passing therethrough a regeneration gas at a temperature between about 80xc2x0 C. and 400xc2x0 C.
The invention includes a system for removing a first and a second minor component from a gas mixture comprising the first and second minor components and one or more major components, which system comprises:
By (a) an adsorber vessel having a first adsorbent zone containing a first adsorbent material and a second adsorbent zone containing a second adsorbent material, wherein the selectivity of the first adsorbent material for the first minor component relative to the second minor component is greater than the selectivity of the second adsorbent material for the first minor component relative to the second minor component, and wherein the average particle diameter of the first adsorbent material and the average particle diameter of the second adsorbent material are substantially the same;
(b) an inlet for passing the gas mixture into the adsorber vessel such that the gas mixture passes through the first adsorbent zone and subsequently through the second adsorbent zone; and
(c) an outlet for withdrawing from the adsorber vessel a purified gas containing the one or more major components and depleted in the first and second minor components.
The first minor component may be nitrous oxide and the second minor component may be carbon dioxide. The one or more major components may comprise oxygen and nitrogen. The gas mixture may comprise air.
The average particle diameter of the first adsorbent material preferably is between about 85% and about 115% of the average particle diameter of the second adsorbent material. The average particle diameter of the first adsorbent material may be between about 0.5 mm and about 5 mm. The first adsorbent material may comprise CaX zeolite and the second adsorbent material may comprise 13X zeolite.
The gas mixture may further comprise water and an additional adsorbent zone may be provided prior to the first adsorbent zone, wherein the additional adsorbent zone preferably contains adsorbent material which selectively adsorbs water from the gas mixture prior to the first adsorbent zone.